XI NORTH AMERICA — CALIFORNIA 34I 



of the Unionidae may be gathered from the fact that Wetherby 

 states^ that in 1874 no less than 832 species in all had been 

 described. 



The entire Mississippi basin is inhabited b}" a common 

 assemblao-e of Unionidae, and a considerable number of the 

 species are distributed over the whole of this area, Texas, and 

 parts of E. Mexico. Some species have spread out of this area 

 into Michigan, Canada, the Red River, and Hudson's Bay district, 

 and even into streams in New York which drain into the 

 Atlantic. An entirely different set of forms occupy the great 

 majority of the rivers falling into the Atlantic, the Appalachian 

 Mountains acting as an effective barrier between the two groups 

 of species, which appear to mingle below the southern end of 

 the range. In many cases Unionidae seem to have no difficulty 

 in migrating from river to river, if the distance is not extreme ; 

 they probably are carried across overflowed districts in time of 

 flood.2 



(2) The Californian Sub-region is markedly distinct from 

 the rest of N. America. The characteristic sombre Helices 

 of the Eastern States are almost entirely 

 wanting, and are replaced by Arionta (20 

 sp.), a larger and more varied group, which 

 may have some affinity to Chinese forms. 

 G-lf/2)tosto77ia (1 sp.) is also peculiar. Sele- 

 nites here has its metropolis, and Pristiolma 

 is a remarkable group of small Hyalinia 

 (^Zonifes}^ but the larger forms of the Eastern 

 States are wanting. Several remarkable ^^^ 227. - iieii^ (Ari- 

 and quite peculiar forms of slug occur, o/ita) jideiis Gray, 

 namely, Ariolimax (whose nearest relation is Oregon. 

 Arion\ Prophysaon^ Hemphillia^ and Binneya. There are no 

 land operculates. 



Not more than 15 to 20 species of the Pleuroceridae (sect. 

 Goniohasis) occur west of the Rocky Mountains, and only a single 

 Unio^ 5 Anoclonta^ and 1 Mai^garitana^ which is common to 

 New England. Poiyvpholyx is a very remarkable ultra-dextral 

 form of Limnaea^ apparently akin to the Choanomphalus of 



1 Journ. Cine. Soc. N'at. Hist. iii. p. 317. The number is doubtless susceptible 

 of very considerable reduction, say by one-half at least. 



2 Simpson, Amer. 2iat. xxvii. 1893, p. 354. 



